Fuel ethanol is currently produced from feedstocks such as cornstarch, sugar cane, and sugar beets. However, the production of ethanol from these sources cannot expand much further due to limited farmland suitable for the production of such crops and competing interests with the human and animal food chain. Finally, the use of fossil fuels, with the associated release of carbon dioxide and other products, in the conversion process is a negative environmental impact of the use of these feedstocks
The possibility of producing ethanol from cellulose-containing feedstocks such as agricultural wastes, grasses, and forestry wastes has received much attention due to the availability of large amounts of these inexpensive feedstocks, the desirability to avoid burning or landfilling cellulosic waste materials, and the cleanliness of ethanol as a fuel compared to gasoline. In addition, a byproduct of the cellulose conversion process, lignin, can be used as a fuel to power the cellulose conversion process, thereby avoiding the use of fossil fuels. Studies have shown that, taking the entire cycle into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The cellulosic feedstocks that may be used for ethanol production include (1) agricultural wastes such as corn stover, wheat straw, barley straw, canola straw, oat straw, and soybean stover; (2) grasses such as switch grass, miscanthus, cord grass, and reed canary grass, (3) forestry wastes such as aspen wood and sawdust, and (4) sugar processing residues such as bagasse and beet pulp.
Cellulose consists of a crystalline structure that is very resistant to breakdown, as is hemicellulose, the second most prevalent component. The conversion of cellulosic fibers to ethanol requires: 1) liberating cellulose and hemicellulose from lignin or increasing the accessibility of cellulose and hemicellulose within the cellulosic feedstock to cellulase enzymes, 2) depolymerizing hemicellulose and cellulose carbohydrate polymers to free sugars and, 3) fermenting the mixed hexose and pentose sugars to ethanol.
Among well-known methods used to convert cellulose to sugars is an acid hydrolysis process involving the use of steam and acid at a temperature, acid concentration and length of time sufficient to hydrolyze the cellulose to glucose (Grethlein, 1978, J. Appl. Chem. Biotechnol. 28:296-308). The glucose product is then fermented to ethanol using yeast, and the ethanol is recovered and purified by distillation.
An alternative method of cellulose hydrolysis is an acid prehydrolysis (or pre-treatment) followed by enzymatic hydrolysis. In this sequence, the cellullosic material is first pre-treated using the acid hydrolysis process described above, but at milder temperatures, acid concentration and treatment time. This pre-treatment process is thought to increase the accessibility of cellulose within the cellulosic fibers for subsequent enzymatic conversion steps, but results in little conversion of the cellulose to glucose itself. In the next step, the pre-treated feedstock is adjusted to an appropriate temperature and pH, then submitted to enzymatic conversion by cellulase enzymes.
The hydrolysis of the cellulose, whether by acid or by cellulase enzymes, is followed by the fermentation of the sugar to ethanol, which is then recovered by distillation.
The temperatures typically used for acid hydrolysis or pre-treatment correspond to saturated steam pressures of 160 psig to 665 psig. The addition of sulphuric acid improves the digestion of the cellulose and shortens the time for pre-treatment from 5-30 minutes to 0.1-5 minutes. Achieving and maintaining these conditions requires a highly pressurized, acid-resistant system. U.S. Pat. No. 4,416,648 (Foody) describes equipment and conditions used in steam explosion pre-treatment, in which the feedstock, steam, and sulfuric acid are added to a reaction vessel, known as a steam gun. In the steam gun, steam is added and the steam pressure is increased rapidly to the desired pressure, followed by sudden explosive decompression. Steam explosion with sulfuric acid added has been the standard pre-treatment process for two decades. It produces pre-treated material that is uniform, has most of the hemicellulose hydrolyzed to simple sugar, and less cellulase enzyme is subsequently required to hydrolyze the cellulose than other pre-treatment processes.
Although steam explosion and other batch pre-treatment processes prepare feedstocks with highly accessible cellulose, they have inherently low throughput. This requires the use of many steam pre-treatment reactors, which is costly and difficult to operate. Furthermore, it is difficult in batch processes—and in steam explosion in particular—to recover and reuse the steam. The steam requirement to achieve and maintain the feedstock at acid hydrolysis or pre-treatment conditions is high. The loss of this steam after a single treatment represents a significant cost in the ethanol production process.
The development of a continuous pre-treatment process delivering the degree of accessibility of the cellulose from a steam explosion process, and that can be economically operated and maintained, has been the focus of a significant research in the field.
U.S. Pat. No. 4,136,207 (Bender) teaches steam pre-treatment to produce a ruminant feed. The feedstock is saturated with moisture and compacted at 2000 psi to remove air and improve the subsequent penetration of steam. A rotating helical feed screw conveys the compacted feedstock into a barrel. Steam made from a steam generator is fed into the reactor barrel at 200-310 psi. The feedstock proceeds through the barrel, at the end of which is a valve to allow steam and volatiles to escape, and a product valve for treated solids to exit. Steam, which is collected in a byproduct recovery column, is returned to the reactor barrel. However, this process does not allow for dilute acid pre-soaking or leaching of the feedstock, nor does the low moisture content of the feedstock allow for pumping of the feedstock. Furthermore, this process does not provide for the use of sulphuric acid, which is required for rapid reaction and the production of maximum accessibility of the cellulose. The absence of sulphuric acid decreases the quality of the pre-treated material, and requires a pre-treatment reactor of increased size. The nature of the pre-treatment reactor used by Bender may inhibit uniform penetration of steam into the fibers due to the formation of pucks and slivers in the material. In addition, adequate sealing of the high-pressure vessel is a challenge.
U.S. Pat. No. 5,366,558 (Brink) describes a continuous acid hydrolysis process that occurs in several stages. The first stage is a steam treatment in the absence of acid. The material is then mechanically disintegrated to a very small particle size, acidified, and sensitized with oxygen. The sensitized material is then heated with steam for the final hydrolysis reaction. The material is washed countercurrently, with the sugar stream and lignin being the products. As multiple reaction steps at high temperature and pressure are required, the risk of severe degradation of the feedstock increases. This in turn limits the level of cellulose conversion and, thereby, the commercial viability of the process. There is no teaching of steam recovery.
U.S. Pat. No. 5,628,830 (Brink) teaches a steam pre-treatment followed by multiple flashes for steam recovery. The lignocellulosic material is finely ground, subjected to a first acid hydrolysis (pre-treatment) stage, then passed through a disintegrator. The slurry then undergoes multiple flashing stages, with steam being recycled, followed by simultaneous saccharification and fermentation to yield ethanol. However, the feedstock of Brink has a moisture content that is too low to allow pumping of the material, and prevents pre-soaking of the feedstock in dilute acid or leaching of the feedstock. In addition, the pre-treatment method of Brink requires the addition of acid at a point where the acid is present through several pieces of equipment; this in turn requires that each piece of equipment be resistant to acid, which adds considerable expense. Finally, the method of '830 recovers and reuses only a portion of the steam, falling short of recovering the substantial amount of steam required for a cost-effective pre-treatment operation.
U.S. Pat. No. 4,237,226 (Grethlein) teaches a continuous pre-treatment in which cellulosic material is slurried in water to about 5-10% solids. The slurry is heated to the reaction temperature and a concentrated stream of sulfuric acid is injected. The acidified slurry is then heated by the injection of live steam to allow for very rapid heating of the slurry. Rapid cooling quenches of the reaction, by flashing across an orifice or capillary at the outlet to the reactor is described. However, recovery of steam is not described.
U.S. Pat. No. 4,556,430 (Converse) includes a non-aqueous carrier in the feedstock to decrease the amount of water present. However, the carrier must be recovered. As some of the lignin may be dissolved by the carrier, recovery of the lignin and carrier is complex. Furthermore, there is no teaching of steam recovery.
U.S. Pat. No. 4,468,256 (Hinger) discloses a process for the hydrolysis of cellulose in vegetable raw materials. The raw material is comminuted and impregnated with dilute acid, after which excess moisture is removed from the material. The acidified material is preheated using recycled steam and is then fed into a reactor, where it is heated in a heating zone. High pressure steam is injected into the material in a hydrolysis zone. The material is subsequently cooled in a release zone, where pressure is rapidly released; the released steam is recycled to the heating zone. Residual steam remaining in the treated materials exits through a port and is recycled to the initial preheating stage. However, the method of Hinger requires the addition of acid at a point where the acid is present through several pieces of equipment, which requires that each piece of equipment be resistant to acid, adding considerable expense. Further, the feedstock of Hinger has a moisture content that is too low to allow pumping of the acidified material.
At present, methods of improved feedstock pre-treatment prior to enzymatic hydrolysis are not particularly economic. More specifically, known methods often require that several pieces of equipment be resistant to acid, which increases equipment costs. Also, many methods require a high steam usage to heat the water of slurried feedstocks. Others do not use slurried feedstocks, but rather dried or compacted materials that cannot be pumped.